re: Best day trip from London?

Greenwich is nice, esp if you're feeling too "urban" after a few days in London. The river ferries are inexpensive (don't bother with the more expensive tourist boats, the regular river "bus" provides the same view without annoying commentary or too many tourists), the ride is short & scenic. Observatory/Prime Meridian are up a small but steep hill set in the middle of a nice park; beautiful view on a nice day....and the town itself has the usual run of tourist shops. Maritime museum is nice for boat geeks & kids, though it is unairconditioned and sweltering beneath glass skylights on a hot day. We enjoyed Turner's Battle of Trafalgar (painting) immensely, mainly b/c it was set in an air conditioned room w/viewing benches.

I'd go to Oxford over Stonehenge, unless you have a serious fascination w/prehistory or woowoo Druidy stuff. Watch a few (or many) episodes of Inspector Lewis or Morse and you can track down the pubs featured in the series; there are even Morse/Lewis walking tours.